1. Field of the Invention
This invention relates to safety fences designed to redirect or prevent passage of vehicles over prohibited ground and is particularly, but not exclusively, applicable to safety fences used on the sides of roads or central reservations of high-speed carriageways, roads or motorways, or embankments.
2. Review of the Prior Art
It is known that safety fences are available consisting of a number of spaced upright posts to which are clamped a number of tensioned horizontal wire ropes. It has been found that these known wire rope fences may be satisfactory when a vehicle approaches a fence at a relatively large angle of impact exceeding 20.degree. whereas at small angles of impact below approximately 10.degree. the vehicle may tend to spin or roll off the fence with consequent danger to the occupants of the vehicle. It is believed that one of the factors contributing to this hazard is the fact that the ropes are normally clamped to the steel posts by means of `U` bolts or other heavy attachment devices which are strong enough to withstand the collision loading.
The disadvantage of clamping wire ropes to posts is overcome by the invention described in UK Patent No. 1,103,873 in which the tension cables are positioned as a slack fit in vertical slots in posts fixed into the ground. Tensioned cables act as a continuous beam to redirect a colliding vehicle smoothly back on to the roadway.
UK Patent No. 1,103,873 provides for a plurality of ropes supported either in slots in the top of the post or supported in brackets on either side of the post such that the cables are parallel to each other.
The testing of safety fence constructions in accordance with this patent has shown that the penetration is greater than that permitted in certain circumstances. It has also been shown that the release of the cables from the slots, whether in the posts or in the brackets, caused by the post deflection, may give rise to a situation that the cables are released too quickly or too far ahead of an impact point. This led to cables going slack too far ahead of impact and insufficient restraint for the vehicle and a danger that the vehicle will run over cable or cables lying on the ground.
The post for all the wire rope fences previously referred to have a main web and at least one flange with a cross-section, such as an `I` section, with the main web of the section extending transversely in the direction of the cables. The post therefore has its weak axis in the direction of the fence, such that it can be more easily run down.
It is believed that the correct juxtaposition of tension cables and posts in the wire rope safety fence according to the invention met the objectives and the tests laid down by the Ministry of Transport at that time. For over 16 years such a wire rope safety fence has been used on the Pennine section of the M62 motorway and has proved to have had considerable advantages over standard type central reservation barriers, in particular they have prevented build-up of drifting snow.